Ligand exchange of perovskite quantum dots and solar cell devices manufactured using the same

ABSTRACT

The present invention relates to a ligand-exchanged inorganic perovskite quantum dot thin film and a solar cell comprising the same. The inorganic perovskite quantum dot thin film provided in one aspect of the present invention is capable of more effective ligand exchange by directly inserting the organic ligand ion-based salt generating organic ligand ions, and the thin film can be used to prepare solar cells with high power conversion efficiency. Therefore, the thin film of the present invention can contribute to the commercialization of perovskite quantum dot solar cells in the future.

CROSS-REFERENCES TO RELATED APPLICATION

This patent application claims the benefit of priority under 35 U.S.C. §119 from Korean Patent Application No. 10-2019-0042960 filed on Apr. 12,2019, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to ligand exchange of perovskite quantumdots and a solar cell device manufactured using the same.

2. Description of the Related Art

Recently, interest in new and renewable energy is increasing due tosoaring oil prices, depletion of fossil fuels, regulations on carbondioxide discharge, etc. In particular, research and development on solarpower system, a pollution-free and inexhaustible domestically andinternationally. A solar cell is a key device of the solar power systemthat converts sunlight directly into electricity.

Inorganic CsPbX₃ (X=Cl, Br, I) perovskite colloid quantum dots (CQDs)have received considerable attention in the field of photovoltaic, lightemitting diode and laser because of their easy optical properties andexcellent photophysical properties of synthesis, size and configurationadjustment. In particular, CsPbX₃ perovskite quantum dots (CsPbX₃-PQDs)are the highest performing photoelectric cell absorption apparatus witha power conversion efficiency of 13.4%, which is a very interestingtopic in the field of CQD solar cell.

Among them, CsPbI₃ perovskite quantum dots are easily decomposed by thepolar solvent due to the ionic bonding property, so it was difficult toselect the polar anti-solvent during the purification process. CsPbI₃perovskite quantum dots are covered with oleate and oleylammoniumligands as surface ligands. Since oleate and oleviammonium ligands withlong hydrocarbon chain have insulating properties in quantum dot thinfilms, they must be removed to be applied to photoelectric elements.

In the prior art, CsPbI₃ perovskite quantum dots were successfullypurified using methyl acetate (MeOAc) with weak polarity (non-patentreference 1, Quantum dot-induced phase stabilization of α-CsPbI₃perovskite for high efficiency photovoltaics, Science 354 (6308), 92-95(2016)). The key reaction in this ligand exchange process is thehydrolysis of methyl acetate. Methyl acetate reacts with H₂O adsorbed onthe surface of perovskite and is hydrolyzed, resulting in the formationof acetic acid and methanol, and the acetate ions produced by therelease of hydrogen ions from acetic acid displace oleate to bind on thesurface of CsPbI₃ perovskite quantum dots. Oleate is removed throughthis process. The results of research on the implementation of a solarcell having an efficiency of 10.7% using the CsPbI₃ perovskite quantumdot thin film as a photoactive layer have been reported.

In another prior art, formamidinium iodide (FAI) cost-treatment wasintroduced for the removal or oleate as well as oleyl ammonium ligands(non-patent reference 2, Enhanced mobility CsPbI₃ quantum dot arrays forrecord-efficiency, high-voltage photovoltaic cells, Sci. Adv. 3 (10),eaao4204 (2017)). Formamidinium ions (FA⁺) generated through FATpost-treatment are bound by substituting oleylammonium on the surface ofCsPbI₃ perovskite quantum dots. Improvements in efficiency up to 13.4%by removing oleylammonium to improve the charge mobility of the CsPbI₃perovskite quantum dot thin film have been reported.

However, acetic acid and methanol are produced during the hydrolysis ofmethyl acetate, and perovskite, which has poor chemical stability due toionic bonding, may be decomposed in an acidic or alcoholic atmosphere.In addition, the FAI post-treatment improves the efficiency of a solarcell, but decreases the open voltage of the solar cell deviceperformance, resulting in large energy loss and shortening the lifespanof the solar cell as the stability deteriorates due to the supply of FA,the organic material. MgF₂ anti-reflection coating is necessary torealize a high efficiency quantum dot solar cell exceeding PCE 13%through the conventional technique using methyl acetate and FAI. Theintroduction of such an anti-reflection coating process is an additionalobstacle to future commercialization because it requires an additionalprocess cost. As another conventional technique, a technique using leadacetate (Pb(OAc)₂) has been reported, but when lead acetate is used,Pb²⁺ acts as a precursor to grow perovskite crystals. This causes aproblem that the perovskite crystals grow and the CsPbI₃ perovskitequantum dots are agglomerated in a needle shape. In addition, theefficiency of the solar cell is reduced because a large amount of —OH isgenerated.

SUMMARY OF THE INVENTION

In an aspect of the present invention, it is an object of the presentinvention to provide an inorganic perovskite quantum dot thin film inwhich surface ligands are exchanged by anions generated by dissociationof alkali metal salt.

In another aspect of the present invention, it is an object of thepresent invention to provide a solar cell with an excellentphotoelectric conversion efficiency comprising the inorganic perovskitequantum dot thin film in which surface ligands are exchanged.

In another aspect of the present invention, it is an object of thepresent invention to provide a preparation method of the inorganicperovskite quantum dot thin film comprising the step of exchanging thequantum dot surface ligands effectively using alkali metal salt.

To achieve the above objects, in an aspect of the present invention, thepresent invention provides an inorganic perovskite quantum dot thin filmcomprising:

an inorganic perovskite quantum dot layer; and

a surface treated layer placed on the surface of the quantum dot layerand treated with the ligand exchange solution containing the ionic saltrepresented by formula 1 below.R¹COO⁻A⁺  [Formula 1]

(In formula 1,

R¹ is C₁₋₅ straight or branched alkyl; and

A⁺ is Na⁺ or K⁺).

In another aspect of the present invention, the present inventionprovides a solar cell comprising the inorganic perovskite quantum dotthin film.

In another aspect of the present invention, the present inventionprovides a preparation method of the inorganic perovskite quantum dotthin film comprising the following steps:

forming an inorganic perovskite quantum dot layer; and

treating the surface of the quantum dot layer with a ligand exchangesolution containing the ionic salt represented by formula 1, andreplacing the surface ligand with the ions represented by formula 2generated by dissociation of the salt of formula 1.R¹COO⁻A⁺  [Formula 1]

(In formula 1,

R¹ is C₁₋₅ straight or branched alkyl; and

A⁺ is Na⁺ or K⁺);R²COO⁻  [Formula 2]

(In formula 2,

R² is C₁₋₅ straight or branched alkyl).

Advantageous Effect

The inorganic perovskite quantum dot thin film provided in one aspect ofthe present invention is capable of more effective ligand exchange bydirectly inserting the organic ligand ion-based salt capable ofminimizing hydrolysis reaction to exchange surface ligands of perovskitequantum dots having insulating properties. The thin film can be used toprepare solar cells with high photoelectric conversion efficiency.Therefore, the thin film can contribute to the commercialization ofperovskite solar cells in the future.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a diagram showing the process of LbL (layer by layer)producing CsPbI₃-PQDs, solid-state ligand exchange reaction.

FIG. 2A is a diagram showing the result of high resolution transmissionelectron microscopy of CsPbI₃-PQDs (as-cast). FIG. 2B is a diagramshowing the result of high resolution transmission electron microscopyof CsPbI₃-PQDs (Pb(OAc)₂ treatment). FIG. 2C is a diagram showing theresult of high resolution transmission electron microscopy ofCsPbI₃-PQDs(Pb(NO₃)₂ treatment). FIG. 2D is a diagram showing the resultof high resolution transmission electron microscopy of CsPbI₃-PQDs(NaOAc treatment).

FIG. 3A is a diagram showing the results of X-ray diffraction ofCsPbI₃-PQD films treated with excess Pb(OAc)₂/Pb(OAc)₂, NaOAc, Pb(NO₃)₂.FIG. 3B is a diagram showing the results of X-ray diffraction ofCsPbI₃-PQD films treated with Pb(NO₃)₂ and excess Pb(NO₃)₂ FIG. 3C is adiagram showing the results of X-ray diffraction of CsPbI₃-PQD filmstreated with NaOAc and excess NaOAc.

FIG. 4A is a diagram showing the results of X-ray photoelectron analysisof CsPbI₃-PQDs treated with neat MeOAc, Pb(NO₃)₂, NaOAc, Pb(OAc)₂, FIG.4B is a diagram showing the FT-IR spectrum of CsPbI₃-PQDs treated withneat MeOAc, Pb(NO₃)₂, NaOAc, Pb(OAc)₂, and FIG. 4C is a diagram showingthe PL spectrum of CsPbI₃-PQDs treated with neat MeOAc, Pb(NO₃)₂, NaOAc,Pb(OAc)₂.

FIG. 5A and FIG. 5B are diagrams showing the results of ultravioletphotoelectron spectroscopy of CsPbI₃-PODs treated with neat MeOAc,Pb(NO₃)₂, NaOAc, Pb(OAc)₂, and FIG. 5C is a diagram showing the energyhand diagram of CsPbI₃-PQDs treated with neat MeOAc, Pb(NO₃)₂; NaOAc,Pb(OAc)₂, compact TiO₂ and Spiro-MeOTAD.

FIG. 6A is a diagram showing the scanning electron microscope image ofCsPbI₃-PQD solar cell. FIG. 6B is a diagram showing the J-V curve ofCsPbI₃-PQD solar cell, and FIG. 6C is a diagram showing the externalquantum efficiency spectrum of CsPbI₃-POD solar cell.

FIG. 7 is a diagram showing the J-V curve and the parameters of theCsPbI₃-POD solar cells of Example 2 and Comparative Example 2.1post-treated with FAI, respectively.

FIG. 8 is a graph showing the of the J-V curve CsPbI₃-PQD solar cells ofExample 2 and Comparative Examples 2.3 to 2.6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is described in detail.

The embodiments of this invention can be modified in various otherforms, and the scope of the present invention is not limited to theembodiments described below. It is well understood by those in the artwho has the average knowledge on this field that the embodiments of thepresent invention are given to explain the present invention moreprecisely. In addition, the “inclusion” of an element throughout thespecification does not exclude other elements, but may include otherelements, unless specifically stated otherwise.

As mentioned above, perovskite covered with ligands with longhydrocarbon chains on the surface must be removed due to the insulatingproperties of the surface ligands. Thus, there is a prior art thatligands were successfully exchanged using methyl acetate. However,methanol and acetic acid are produced during the hydrolysis of methylacetate. And there is a problem that the ion-binding perovskite has ahigh risk of decomposition in acid or alcohol environments. As anotherconventional technique, a technique using lead acetate (Pb(OAC)₂) hasbeen reported, but when lead acetate is used, Pb²⁺ acts as a precursorto grow perovskite crystals. This causes a problem that the perovskitecrystals grow and the CsPbI₃ perovskite quantum dots are agglomerated ina needle shape. In addition, the efficiency of the solar cell is reducedbecause a large amount of —OH is generated.

Thus, in one aspect of the present invention, a ligand exchange solutioncomprising alkali metal cation-based salt that can provide ligandsdirectly without undergoing hydrolysis is provided in order to solve theproblem of surface ligand exchange of the perovskite quantum dots. As aresult of exchanging the surface ligand of the quantum dot thin film bythe ligand exchange solution, it was found that the long carbon chainligand on the surface was effectively replaced with the short carbonchain ligand. In addition, the solar cell using the quantum dot thinfilm with the exchanged surface ligand as a photoactive layer canincrease the power conversion efficiency up to 12.4%, so that a highlyefficient solar cell can be prepared using the quantum dot thin filmprovided in one aspect of the present invention.

In an aspect of the present invention, the present invention provides aninorganic perovskite quantum dot thin film comprising:

an inorganic perovskite quantum dot layer; and

a surface treated layer placed on the surface of the quantum dot layerand treated with the ligand exchange solution containing the ionic saltrepresented by formula 1 below.R¹COO⁻A⁺  [Formula 1]

(In formula 1,

R¹ is C₁₋₅ straight or branched alkyl; and

A⁺ is Na⁺ or K⁺).

In the inorganic perovskite quantum dot thin film according to thepresent invention, the surface treated layer is characterized in thatthe ligands are exchanged by the ions represented by formula 2 generatedby dissociation of the ionic salt represented by formula 1.R²COO⁻  [Formula 2]

(In formula 2,

R² is C₁₋₅ straight or branched alkyl).

In the inorganic perovskite quantum dot thin film according to thepresent invention, the surface treated layer can be formed bysolid-state ligand exchange (SLE) reaction.

Particularly, the solid-state ligand exchange reaction can be performedon the quantum dot thin film.

The inorganic perovskite quantum dots provided in one aspect of thepresent invention can be the perovskite quantum dots represented byformula 3 below.BCX₃  [Formula 3]

In formula 3,

B is one element of Cs, Rh, Ba, In, K and Ti, C is one element of Pb,Sn, Bi, Ag, Ge and Zr, and X is one element of F, Cl, Br and I.

The perovskite quantum dots represented by formula 3 can be CsPbI₃perovskite quantum dots.

In the perovskite quantum dot thin film provided in another aspect ofthe present invention, the surface treated layer can be treated withformamidinium iodide (FAI) after treatment with the ligand exchangesolution containing the ionic salt represented by formula 1 below.R¹COO⁻A⁺  [Formula 1]

(In formula 1,

R¹ is C₁₋₅ straight or branched alkyl; and

A⁺ is Na⁺ or K⁺).

In the perovskite quantum dot thin film provided in another aspect ofthe present invention, the ionic salt represented by formula 1 isCH₃COO⁻Na⁺ (sodium acetate).

In another aspect of the present invention, the present inventionprovides a solar cell comprising the inorganic perovskite quantum dotthin film.

The solar cell provided in one aspect of the present invention caninclude a quantum dot thin film in which surface ligands are exchangedby short carbon chains as a photoactive layer. The solar cell has a highcurrent density capable of obtaining a power conversion efficiency of12.4%. In addition, the solar cell comprising the quantum dot thin filmtreated with formamidium iodide (FAI) for additional ligand exchange asa photoactive layer exhibits excellent power conversion efficiency of upto 13.3%. Such experimental results have been described in detail in thefollowing experimental examples.

In another aspect of the present invention, the present inventionprovides a preparation method of the inorganic perovskite quantum dotthin film comprising the following steps:

forming an inorganic perovskite quantum dot layer; and

treating the surface of the quantum dot layer with a ligand exchangesolution containing the ionic salt represented by formula 1, andreplacing the surface ligand with the ions represented by formula 2generated by dissociation of the salt of formula 1.R¹COO⁻A⁺  [Formula 1]

(In formula 1,

R¹ is C₁₋₅ straight or branched alkyl; and

A⁺ is Na⁺ or K⁺).R²COO⁻  [Formula 2]

(In formula 2,

R² is C₁₋₅ straight or branched alkyl).

Hereinafter, the preparation method of the inorganic perovskite quantumdot thin film provided in an aspect of the present invention will bedescribed in detail step by step.

Hereinafter, the preparation method of the CsPbI₃ perovskite quantum dotthin film as an example of the inorganic perovskite quantum dot thinfilm will be described.

In the preparation method of the inorganic perovskite quantum dot thinfilm provided in an aspect of the present invention, step 1 is to forman inorganic perovskite quantum dot layer.

The quantum dot layer can be formed by spin-coating CsPbI₃-PQD pelletsdispersed in octane (75 mg/mL). The rpm and time of the spin-coating canbe properly adjusted according to the reaction conditions. Preferably,the spin-coating can be performed for 5 to 30 seconds at 1000 to 2000rpm, and the spin-coating can be repeated several times as necessary.

The CsPbI₃-PQD pellets can be prepared by the preparation method of toepreparative example described later. The compounds such as solvents andgases and the conditions such as temperature and reaction time for theCsPbI₃-PQD pellet production can be used without particular limitationas long as the CsPbI₃-PQD pellets can be well separated and obtained.

In the preparation method of the inorganic perovskite quantum dot thinfilm provided in an aspect of the present invention, step 2 is to treatthe surface of the quantum dot layer with a ligand exchange solutioncontaining the ionic salt represented by formula 1, and replace thesurface ligand with the ions represented by formula generated bydissociation of the salt of formula 1.R¹COO⁻A⁺  [Formula 1]

(In formula 1,

R¹ is C₁₋₅ straight or branched alkyl; and

A⁺ is Na⁺ or K⁺).R²COO⁻  [Formula 2]

(In formula 2,

R² is C₁₋₅ straight or branched alkyl).

The step of replacing the surface ligand with the ions represented byformula 2 can be performed by solid-state ligand exchange (SLE)reaction.

Particularly, the solid-state ligand exchange reaction can be performedon the inorganic perovskite quantum dot thin film.

The preparation method of the inorganic perovskite quantum dot thin filmcan be performed by the LBL (layer by layer) method in which each stepis repeatedly performed two to eight times. It Is possible to form afilm having a desired thickness through the LBL method, preferably to athickness of about 300 nm.

In another aspect of the present invention, the salt of formula 1 isCH₃COO⁻Na⁺ (sodium acetate).

The inorganic perovskite quantum dot thin film provided in one aspect ofthe present invention provides ions directly without hydrolysisreaction, so that long carbon chain ligands on the surface can besubstituted with short carbon chain ligands very efficiently. Inaddition, a solar cell comprising the quantum dot thin film has a highcurrent density and high power conversion efficiency. Therefore, theinorganic perovskite quantum dot thin film can make a significantcontribution to the commercialization of solar cells, which may bedirectly supported by the following examples and experimental examples.

Hereinafter, the present invention will be described in detail by thefollowing examples.

However, the following examples are only for illustrating the presentinvention, and the contents of the present invention are not limitedthereto.

Preparative Example 1 Preparation of Colloidal CsPbI₃-PQDs

To prepare an solution of Cs-oleate, CsCO₃ (0.4078 g), 1-octadecene (20ml) and oleic acid (1.25 were placed in a 100 mL, 3-neck round flask,followed by degassing at 120° C. for 30 minutes in vacuum. The flask wasfilled with N₂ gas and the gas was removed again. An solution ofCs-oleate was prepared by repeating this process three times, which wasstored in N₂.

To prepare CsPbI₃PQDs, PbI₂ (0.5 g) and 1-octadecene (25 ml) were placedin a 100 mL 3-neck round flask, followed by degassing at 115° C. for 30minutes. Pre-degassed oleic acid (2.5 ml) and oleylamine (2.5 mL) wereinjected into the flask, followed by degassing at 115° C. for 30minutes. The flask was filled with N₂ gas, then the temperature wasraised to 185° C., and Cs-oleate (2 mL) was injected rapidly into theflask. After 10 seconds, the flask was quenched with ice water to roomtemperature, and MeOAc (35 ml) was added to the synthesized CsPbI₃-PQDaqueous solution (15 ml) as a polar solvent to separate the desiredCsPbI₃-PQDs, followed by centrifugation at 5000 rpm for 3 minutes. Afterremoving the supernatant, the precipitated CsPbI₃-PQDs were dispersed inhexane (5 mL) to which MeOAc (7 mL) was added. The solution wascentrifuged at 5000 rpm for 3 minutes to remove the supernatant. Theprecipitated CsPbI₃-PQDs were dispersed in hexane (15 mL), followed bycentrifugation at 5000 rpm for 3 minutes to remove the aggregatedCsPbI₃-PQDs. The precipitate was removed and the supernatant wascollected and stored at 4° C. in the dark place for 48 hours. Afterremoving the precipitate, hexane was evaporated in vacuum and the driedCsPbI₃-PQD pellet was dispersed in octane at the concentration of ˜75mg/mL.

Example 1 Preparation of CsPbI₃-PQD Thin Film using Sodium Acetate(NaOAc)

The CsPbI₃-PQD solution (75 mg/mL) in octane prepared in PreparativeExample 1 was spin-coated at 1000 rpm for 20 seconds and at 2000 rpm forseconds. 1 mg mL⁻¹ of NaOAc solution in MeOAc solvent was added to theCsPbI₃-PQD film prepared above, which was spin-dried. Then, the ligandexchanged film was washed with MeOAc and spin-dried. The ligand solutiontreatment and washing processes were performed in an environment inwhich the relative humidity was controlled at 15%˜20% for effectiveligand exchange. This process was repeated 4˜6 times to make the filmhad the desired thickness (˜300 nm).

Example 2 Preparation of Solar Cell Comprising CsPbI₃-PQD Thin Filmusing Sodium Acetate (NaOAc)

In order to wash the patterned-FTO, it was sonicated sequentially indetergent water, deionized water, acetone, isopropyl alcohol for 10minutes each. The washed FTO substrate was exposed to UV to removecontaminants and the surface was made hydrophilic for compact TiO₂coating. A sol-gel derived compact TiO₂ (c-TiO₂) layer, used as ETL insolar cells, was made on the patterned-FTO substrate. The aqueousprecursor solution was purchased from Sharechem for convenience, but theincluded compounds and preparation methods are the same as the reportedsol-gel method of Swarnkar et al. The aqueous precursor solution wasspin-coated on the FTO substrate at 3000 rpm for 30 seconds, and thenthe substrate was annealed at 500° C. for 1 hour. After cooling to roomtemperature, the substrate was immersed in an aqueous solution of TiCl₄(120 mM) at 70° C. for 1 hour. The substrate was washed with deionizedwater and annealed at 500° C. for 1 hour. TiCl₄ treatment improves thequality of the interface of the c-TiO₂ layer.

A CsPbI₃-PQD thin film layer was prepared on the FTC/c-TiO₂ substrate bythe method of Example 1 for the preparation of CsPbI₃-PQD thin filmprepared via solid-state ligand exchange.

The spiro-MeOTAD layer used as HTL in solar cells was spin-coated at4000 rpm for 30 seconds. The spiro-MeOTAD solution was prepared by thefollowing method. Spiro-MeOTAD (72.3 mg), chlorobenzene (1 mL),2-amylpyridine (28.8 μL) and Li-TFSI (17.5 μL) solution, which isdissolved in acetonitrile at the concentration of 520 mg/mL, are mixed.MoO_(x) and a metal electrode are fabricated to a thickness of 15 nm and120 nm, respectively, using a thermal evaporator.

Comparative Example 1

1.1. Preparation of CsPbI₃-POD Thin Film Using Methyl Acetate (MeOAc)

The CsPbI₃-PQD solution (75 mg/mL) in octane prepared in PreparativeExample 1 was spin-coated at 1000 rpm for 20 seconds and at 2000 rpm for5 seconds. A MeOAc solution was dropped on the CsPbI₃-PQD film preparedabove, which was spin-dried. This process was performed in anenvironment in which the relative humidity was controlled at 15%˜20% foreffective ligand exchange. This process was repeated 4˜6 times to makethe film had the desired thickness (˜300 nm).

1.2. Preparation of CsPbI₃-PQD Thin Film Using Lead Nitrate (Pb(NO₃)₂)

A CsPbI₃-PQD thin film was prepared in the same manner as described inExample 1, except that a lead nitrate (Pb(NO₃)₂) solution in MeOAcsolvent was dropped instead of a NaOAc solution in MeOAc solvent.

1.3 Preparation or CsPbI₃-PD Thin Film Using Lead Acetate (Pb(OAc)₂)

CsPbI₃ thin film was prepared in the same manner as described in Example1, except that a lead acetate (Pb(OAc)₂) solution in MeOAc solvent wasdropped instead of a NaOAc solution in MeOAc solvent.

Comparative Example 2

2.1 Preparation of Solar Cell Comprising CsPbI₃-PQD Thin Film Using LeadNitrate (Pb(NO₃)₂)

A CsPbI₃-POD thin film was prepared in the same manner as described inExample 2 except that a CsPbI₃-PQD thin film layer was prepared by themethod of Comparative Example 1.2 instead of the method of Example 1.

2.2. Preparation of Solar Cell Comprising CsPbI₃-PQD Thin Film UsingLead Acetate (Pb(OAc)₂)

A CsPbI₃-PQD thin film was prepared in the same manner as described inExample 2 except that a CsPbI₃-PQD thin film layer Was prepared by themethod of Comparative Example 1.3 instead of the method of Example 1.

2.3. Preparation of Solar Cell Comprising CsPbI₃-PQD Thin Film UsingLithium Acetate (LiOAc)

A CsPbI₃-PQD thin film was prepared in the same manner as described inExample 2 except that a LiOAc solution in MeOAc solvent was used insteadof a NaOAc solution in MeOAc solvent for the production of a CsPbI₃-PQDthin film layer by the method of Example 1.

2.4. Preparation of Solar Cell Comprising CsPbI₃-PQD Thin Film UsingPotassium Acetate (KOAc)

A CsPbI₃-PQD thin film was prepared in the same manner as described inExample 2 except that a KOAc solution in MeOAc solvent was used insteadof a NaOAc solution in MeOAc solvent for the production of a CsPbI₃-PQDthin film layer by the method of Example 1.

2.5. Preparation of Solar Cell Comprising CsPbI₃-PQD Thin Film UsingRubidium Acetate (RhOAc)

A CsPbI₃-PQD thin film was prepared in the same manner as described inExample 2 except that an RhOAc solution in MeOAc solvent was usedinstead of a NaOAc solution in MeOAc solvent for the production of aCsPbI₃-PQD thin film layer by the method of Example 1.

2.6. Preparation of Solar Cell Comprising CsPbI₃-PQD Thin Film UsingCesium Acetate (CsOAc)

A CsPbI₃-PQD thin film was prepared in the same manner as described inExample 2 except that a CsOAc solution in MeOAc solvent was used insteadof a NaOAc solution in MeOAc solvent for the production of a CsPbI₃-PQDthin film layer by the method of Example 1.

Experimental Example 1 Analysis of CsPbI₃ Perovskite Quantum Dot ThinFilm Properties

In order to analyze the properties of the ligand exchanged CsPbI₃perovskite quantum dot thin films using a ligand exchange solution, thefollowing experiments were performed using the CsPbI₃-PQD thin filmsprepared in Example 1 and Comparative Example 1.

1.1. High-Resolution Transmission Electron Microscopy

To confirm the effect of ionic salts dissolved in MeOAc on CsPbI₃-PQDsduring ligand exchange, high-resolution transmission electron microscopy(HR-TEM) was conducted. The CsPbI₃-PQDs dispersed in octane were loadedonto a TEM copper lattice grid and observed under a microscope. Then, ansolution of 1 mg mL⁻¹ MeOAc containing Pb(NO₃)₂, NaOAc and Pb(OAc)₂ wastreated for ligand exchange of the CsPbI₃-PQDs on the TEM grid. Theresults are shown in FIG. 2A to 2D.

The thin film of Comparative Example 1.3 did not have good surfacemorphology of CsPbI₃-PQD solid due to aggregation and fusion of theCsPbI₃-PQDs. This suggests that Pb(OAc)₂ can act as a lead precursor forthe growth of CsPbI₃ perovskite crystals during ligand exchange. It wasconfirmed that the thin films of Example 1 and Comparative Example 1.2had a larger ratio of empty spaces. This is because the distance betweenCsPbI₃-PQDs was reduced by replacing the long chain of oleate with theshort chain of OAc anions while preserving the CsPbI₃-PQD size. Inparticular, the thin film of Example 1 exhibited the largest emptyspace, which showed that the ligand exchange efficiency by NaOAc ofExample 1 was the best.

1.2. X-Ray Diffraction Analysis

X-ray diffraction (XRD) analysis was performed to confirm the fusion ofCsPbI₃-PQDs. The results are shown in FIG. 3A to 3C.

When the CsPbI₃-PQD solid was treated with excess Pb(OAc)₂, thediffraction peaks corresponding to (100) and (200) were increasedapproximately twice than those of Comparative Example 1.3 (FIG. 3A).However, treatment with excess Pb(NO₃)₂ or NaOAc did not affect thegrowth of CsPbI₃-PQD crystals during ligand exchange (FIGS. 3B and 3C).From the results above, it was confirmed that NaOAc could be used as theOAc⁻-based ionic salt capable of directly generating OAc ions duringligand exchange of CsPbI₃-PQDs.

1.3. X-Ray Photoelectron Spectroscopy

X-ray Photoelectron spectroscopy (XPS) Was conducted to confirm thechemical composition and electron state before and after ligand exchangein the films of Example 1 and Comparative Examples 1.1 to 1.3. Theresults are shown in FIG. 4A and Table 1. Binding energy was correctedusing C is peak at 284.5 eV. The element composition and peak intensitywere standardized using the measured values of Pb element and themaximum intensity of Pb 3d_(5/2) peak, respectively.

The non-ligand exchanged CsPbI₃-POD thin film showed slightly higherCs/Pb and I/Pb ratios than 1 and 3, respectively. This is because asignificant portion of the CsPbI₃ perovskite is cube-shapednanocrystals, about 10 nm in size, whose surface ends with Cs— or I—.Ligand exchange of long hydrocarbon chains of oleate ligands of theCsPbI₃-PQD with short chains of OAc ions led to a significant reductionof the C/Pb ratio. The I/Pb ratio of the film of Comparative Example 1.3was less than 3 and the C/Pb ratio thereof was the smallest. Throughthis, it was confirmed that the I-deficient surface due to the growth ofCsPbI₃ perovskite crystals and the reaction of excess I with Pb(OAc)₂remained on the CsPbI₃-PQD surface. The I/Pb ratio of the films ofExample 1, Comparative Example 1.1 and Comparative Example 1.2 was 3 ormore, indicating that the CsPbI₃-PQD size was maintained. Among thefilms of Example 1, Comparative Example 1.1 and Comparative Example 1.2in which the nanocrystal size of CsPbI₃-PQD was maintained, the C/Pbratio of the film of Example 1 was the smallest. Thereof, it wasconfirmed that NaOAc treatment was very effective in removing the longchains of oleate ligands connected to the surface of CsPbI₃-PQDs.

TABLE 1 Comparative Comparative Comparative Example Example ExampleAs-cast 1.1 1.2 1.3 Example 1 Cs 3d_(5/2) 1.04 1.14 1.13 1.09 1.14 Pb4f_(7/2) 1.00 1.00 1.00 1.00 1.00 I 3d_(5/2) 3.20 3.21 3.15 2.91 3.25 C1s 8.08 4.48 4.27 3.48 3.93 O 1s 0.49 0.93 0.65 1.08 0.48

To investigate the oxygen signal origin in detail, O 1s spectra weredeconvolutiond into two peaks corresponding to different chemicalspecies. The results are shown in FIG. 4A. The O 1s peak at 532.5 eV wasoriginated from the carboxylate group (COO⁻) derived from the oleate andOAc ions bound to the surface of CsPbI₃-PODs. and the peak at 531.2 eVwas originated from the metal hydroxide (ie Pb—OH) derived from themethanol produced by hydrolysis of MeOAc.

The film of Example 1 showed the lowest contribution at 531.2 eV,indicating that Pb—OH was formed in the smallest amount after ligandexchange. The Pb—OH formation led to sub-bandgap trap states in PbS CQDsand low charge transport of lead halide perovskite, resulting in poorphotovoltaic cell performance. In addition, polar solvents, such asalcohols, induced lattice distortion of the cube-shaped CsPbI₃ nanocube,leading to a phase change into the orthorhombic structure. The O/Pbratio and COO⁻/OH ratio of the thin film of Example 1 were almost thesame as those of the as-cast CsPbI₃-PQD thin film stabilized by oleateand oleylammonium ligands in which the ligand exchange reaction did notundergo. This showed low surface trap states and high photoluminescencequantum yields (PLQYs).

These results indicate that NaOAc treatment can maintain the surfacetrap state unique to the CsPbI₃-PQDs thin film without cubic phaseinstabilization andhydrolysis of MeOAc, which can be negatively affectedby oxygen species. The highest O 1s peak of the film of ComparativeExample 1.3 was shifted to a low binding energy (531.5 eV) and the Pb—OHsignal was significantly increased, indicating that the surface Pb—OHbond was more dominant than COO⁻.

1.4. FT-IR Spectrum Analysis

FT-IR spectrum of the CsPbI₃-PQDs before and after ligand exchange wasanalyzed. The results are shown in FIG. 4R.

The CsPbI₃-PQD thin film without ligand exchange showed IR signals ofaliphatic hydrocarbon chains (1380, 1470, 2850, 2920, 2960 cm⁻¹), COO⁻(1410, 1560, 1710 cm⁻¹) and ammonium (3000-3200 cm⁻¹) originated fromoleic acid and oleylammonium ligands. The thin films of Example 1 andComparative Examples 1.1 to 1.3 showed reduced aliphatic IR signals. Inparticular, the thin film of Comparative Example 1.3 showed anadditional IR signal at 3650 cm⁻¹, which corresponds to the O—Hstretching peak and Pb—OH bending peak of the metal hydroxide formedduring the growth of CsPbI₃ perovskite crystals. The thin film ofExample 1 exhibited lower IR signals of aliphatic hydrocarbon chainsthan the thin film of Comparative Example 1.2. This showed that NaOAcremoved long chains of oleate more effectively.

1.5. Photoluminescence Spectrum Analysis

Photoluminescence (PL) spectrum of the CsPbI₃-PQDs before and afterligand exchange was analyzed. The results are shown in FIG. 4C and table2.

The Pt spectrum of the CsPbI₃-PQD was red-shifted from 680 nm to 683 nmas the distance between nanocrystals was decreased due to the removal ofoleic acid ligands by treatment of the ligand exchange solution. Inaddition, the thin film of Example 1 exhibited the narrowest full-widthat half-maximum (FWHM) of 36.9 nm, indicating that the CsPbI₃-PQDs weremore effectively maintained in low aggregation and surface trap statesduring ligand exchange. The PL spectrum of the thin film of ComparativeExample 1.3 showed a wider FWHM (41.4 nm) and red-shift to 685 nm,indicating that it was much more fused than other CsPbI₃-PQDs andremained in the surface trap state.

TABLE 2 Comparative Comparative Comparative Example Example ExampleAs-cast 1.1 1.2 1.3 Example 1 PL peak 680 683 683 685 683 (nm) FWHM 37.538.8 38.6 41.4 36.9 (nm)

1.6. Ultraviolet Photoelectron Spectroscopy

Ultraviolet photoelectron spectroscopy (UPS) was conducted to confirmthe energy levels of the CsPbI₃-PQDs affecting photovoltaic cell actionbefore and after ligand exchange. The results are shown in FIG. 5A to5C.

The Fermi energy (E_(F)) and valence band energy (E_(V)) levels weredetermined by cutoff energy (E_(cutoff)) in the high binding energyregion and linear extrapolation fa the low binding energy region,respectively. An energy level diagram of the CsPbI₃-PQD solar cell isshown in FIG. 5C. Compared with the energy levels of c-TiO₂ andSpiro-MeOTAD, the thin film of Example 1 showed an upward shift at theconduction band minimum (CBM) and valence band maximum (VBM) levels.This indicates that the charge extraction, such as electrons and holes,is more energetic in the thin film of Example 1 than the thin films ofComparative Examples 1.2 and 1.3.

Experimental Example 2 Analysis of Solar Cell Solar Performance

To confirm the performance of a solar cell comprising the CsPbI₃-PQDthin film using sodium acetate (NaOAc) according to the presentinvention, the power conversion efficiency of the solar cells preparedin Example 2 and Comparative Example 2 was analyzed.

2.1. Scanning Electron Microscopy

Scanning electron microscopy (SEM) images of the solar cells preparedExample 2 and Comparative Examples 2.1 and 2.2 were analyzed. Theresults are shown in FIG. 6A.

CsPbI₃-PQDs were prepared on the c-TiO₂ layer as an electron transferlayer (ETL). Then, Spiro-OMeTAD was used as a hole transfer layer (HTL),and MoO_(x)/Ag was prepared as an upper electrode.

2.2. Analysis of J-V Curves and External Quantum Efficiency

Current density-voltage (J-V) curves, detailed device parameters andexternal quantum efficiency (EQE) spectra of the solar cells of Example2, Comparative Example 2.1 and Comparative Example 2.2 were analyzed.The results are shown in FIGS. 6B and 6C and table 3.

The solar cell of Comparative Example 2.1 showed 10.7% of PCE, and thesolar cell of Example 2 showed the highest POE of 12.4%. This is due tothe ligand exchange process that generated OAc ions directly from NaOAcsalts to replace the long chain of oleic acid ligand with the shortchain of OAc ions. In particular, the improvement of J_(sc) and FF ofthe solar cell of Example 2 was strongly related to the enhanced chargetransport capability of CsPbI₃-PODs as well as the favorable bandalignment of the solar cell device. However, the solar cell ofComparative Example 2.2 showed low efficiency due to poor fusion ofCsPbI₃-PQDs and surface morphology. Specifically, the power conversionefficiency of the solar cell of Example 2 was about 16% higher than thatof the solar cell of n Comparative Example 2.2, the open voltage of thesolar cell of Example 2 was about 42% higher than that of the solar cellof Comparative Example 2.1, and the power conversion efficiency of thesolar cell of Example 2 was also about 249% higher than that of thesolar cell of Comparative Example 2.1.

Therefore, it was confirmed that the efficiency of the solar cell usingthe CsPbI₃-PQD thin film ligand-exchanged with a ligand solutioncontaining NaOAc ion salts as an photovoltaic absorber layer wasexcellent.

TABLE 3 V_(OC) (V) J_(SC) (mA/cm²) FF (%) PCE (%) Example 2 1.21 14.769.6 12.4 Comparative 1.19 13.9 65.3 10.7 Example 2.1 Comparative 0.859.83 42.5 3.55 Example 2.2

2.3. Analysis of Power Conversion Efficiency

Additional FAI post-treatment was performed on the thin films of Example1 and Comparative Example 1.3 to replace the oleylammonium ligands withthe short chains of FA cations. The power conversion efficiency at thistime was investigated. The results are shown in FIG. When Post-treatmentwas performed on the thin film of Example 1, PCE was 13.3% without theaid of MgF₂ antireflective coating, and when FAI post-treatment wasperformed on the thin film of Comparative Example 1.3, PCE was 12.4%.

From the above results, it was confirmed that NaOAc removed the oleateligands very efficiently, and enhanced the electronic coupling ofCsPbI₃-PQDs.

To evaluate the power conversion efficiency of the solar cells usingacetate salts containing various alkali metal cations, the powerconversion efficiency of the solar cells of Example 2 and ComparativeExamples 2.3 to 2.6 was investigated. The results are shown in FIG. 8and table 4.

TABLE 4 V_(OC) (V) J_(SC) (mA/cm²) FF (%) PCE (%) Example 2 1.20 12.966.2 10.2 Comparative 0.19 12.2 28.8 0.67 Example 2.3 Comparative 1.1912.5 67.0 9.94 Example 2.4 Comparative 0.96 7.14 31.4 2.16 Example 2.5Comparative 0.99 13.1 39.9 5.20 Example 2.6

The power conversion efficiency of the solar cell of Example 2 using Nacations was the highest, which was 10.2%, and the power conversionefficiency of the solar cell of Comparative Example 2.4 using K cationswas also high as 9.94%. On the other hand, the solar cells ofComparative Examples 2.3, 2.5 and 2.6 using Li cations, Rb cations andCs cations showed low power conversion efficiency of 0.67%, 2.16% and5.20%, respectively. Therefore, it was confirmed that the efficiency ofthe CsPbI₃-PQD solar cell comprising acetate containing Na cations wasthe best.

As mentioned above, the present invention has been described in detailthrough the preferred preparative examples, examples and experimentalexamples, but the scope of the present invention is not limited to thespecific examples, and should be interpreted by the appended claims. Inaddition, those of ordinary skill in the art should understand that manymodifications and variations are possible without departing from thescope of the present invention.

What is claimed is:
 1. A preparation method of an inorganic perovskitequantum dot thin film comprising the following steps: forming aninorganic perovskite quantum dot layer; and treating the surface of thequantum dot layer with a ligand exchange solution containing an ionicsalt represented by formula 1, and replacing a surface ligand with ionsrepresented by formula 2 generated by dissociation of the ionic salt offormula 1:R¹COO⁻A⁺  [Formula 1] (In formula 1, R¹ is C₁₋₅ straight or branchedalkyl; and A⁺ is Na⁺ or K⁺)R²COO⁻  [Formula 2] (In formula 2, R² is C₁₋₅ straight or branchedalkyl).
 2. The preparation method of an inorganic perovskite quantum dotthin film according to claim 1, wherein the step of replacing thesurface ligand with the ions represented by formula 2 is performed bysolid-state ligand exchange (SLE) reaction.
 3. The preparation method ofan inorganic perovskite quantum dot thin film according to claim 1,wherein the step of forming the inorganic perovskite quantum dot layeris performed by using a perovskite quantum dot solution.
 4. Thepreparation method of an inorganic perovskite quantum dot thin filmaccording to claim 1, wherein the steps are repeatedly performed two toeight times.
 5. The preparation method of an inorganic perovskitequantum dot thin film according to claim 2, wherein the ionic saltrepresented by formula 1 is CH₃COO—Na⁺ (sodium acetate).